The present invention relates to a hydraulic control device for an automatic transmission.
Generally, in a multi-stage automatic transmission installed in vehicles, each shift speed is achieved by controlling the rotating state of each rotating element included in a speed change gear mechanism based on the engaged state of a plurality of friction engagement elements. The engaged state of these friction engagement elements is controlled by electrically adjusting the engaging pressure using a solenoid valve and supplying the engaging pressure to a hydraulic servo of each friction engagement element.
In the automatic transmission, when a failure occurs where no electricity is supplied to the solenoid valve, or a so-called solenoid-all-off state, electrical shift control using the solenoid valve is impossible. The solenoid-all-off state occurs, for example, when the control unit (ECU) is down, battery wiring is broken, and a short-circuit occurs.
To solve this problem, Japanese Patent Application Publication No. JP-A-2001-90829 proposes that, when such solenoid-all-off state occurs during running (in a forward range), two types of shift speeds are achieved in accordance with the shift speed before the occurrence of the solenoid-all-off state without using the electrical pressure regulation control of the solenoid valve, thus securing continuous running performance. More specifically, the device in Japanese Patent Application Publication No. JP-A-2001-90829 is structured such that, if the solenoid-all-off state occurs between a forward first speed and a forward third speed, the forward third speed is achieved, and if the solenoid-all-off state occurs between a forward fourth speed and a forward sixth speed, the forward sixth speed is achieved. Therefore, a low-speed running performance is secured if the solenoid-all-off state occurs during low-speed running, while high-speed running performance is secured if the solenoid-all-off state occurs during high-speed running. The device in Japanese Patent Application Publication No. JP-A-2001-90829 is structured such that forward third speed is achieved if the engine is restarted after the vehicle is stopped and the engine is stopped after the occurrence of the solenoid-all-off state, thereby making it possible to restart the vehicle, which is referred to as limp-home function.
The device disclosed in Japanese Patent Application Publication No. JP-A-2001-90829 is provided with three valves in total: a fail-safe shift valve (VA) to switch the shift speed to achieve two types of shift speeds (forward sixth speed, forward third speed) when the solenoid-all-off state occurs; a high shift speed storage shift valve (VB) to store the state of high speed and output hydraulic pressure to the fail-safe shift valve at a high speed; and a high shift speed cancel shift valve (VC) to cancel the storage state of the high speed in the high shift speed storage shift valve (or four valves if an abnormality detection solenoid valve (SE) to change the signal pressure between normal time and failure time is included).